Imaging sensors or devices based on silicon technology typically require the use of an infrared blocking element somewhere in the optical chain. The purpose of this element is to prevent infrared radiation (IR) or light (typically considered to be light with a wavelength longer than 780 nm) from entering the imaging array. Silicon-based devices will typically be sensitive to light with wavelengths up to approximately 1200 nm. If the IR is permitted to enter the array, the device will respond to the IR, and generate an output image signal. Since the purpose of an imaging system (in this context) is to create a representation of the visible light present in a scene, the IR will introduce a false response and distort the image. In a monochrome (black and white) imaging system, the result can be an obviously distorted rendition. For example, foliage and human skin tones may appear unusually light. In a color imaging system, the introduction of IR will distort the coloration and produce an image with incorrect and de-saturated color.
A common method for preventing these difficulties is to use ionically colored glass or a thin-film optical coating on glass to create an optical element which passes visible light (typically from 380 nm to 780 nm) and blocks the IR. This element can be placed in front of the taking lens, located within the lens system, or it can be incorporated into the imager package. The principle disadvantages to this approach are cost and added system complexity. The cost of an ionically colored glass element can be approximately $1.50 to $2.00 in high volume. Thin film coatings can be implemented at somewhat lower cost (approximately $0.50 to $1.00 in volume), but suffer from the additional disadvantage of exhibiting a spectral shift as a function of angle. Thus, in an imaging system they do not provide a uniform transmittance characteristic from the center of the image field to the edge. Both filter types add to the system complexity by introducing an extra piece-part which must be assembled into the imaging system.
It can therefore be desirable to provide for an imaging system without an IR blocking filter in which IR does not appreciably effect the resulting image signal.